[jdom-interest] Thread safe question: OK for simultaneous read-only access?

bob mcwhirter bob at werken.com
Sat Oct 27 23:49:46 PDT 2001

Reading and traversal are basically const operations (sorry for the
C++ throwback).  They have no side-effects, and thus, there is no
way for one thread to alter another's view of the underlying data.

As soon as you throw even 1 mutating event into the mix, though,
the results quickly become undefined.  Since the underlying storage
within the JDOM classes are from the Java Collections framework,
you'll see 'fail-fast' behaviour should you commit a synchronization
error.  This occurs only within the context of a node, and not 
an entire Document.

If Thread-A is reading Element-1's list of attributes, and Thread-B
alters Element-1's list of attributes simultaneously, you'll witness
an exception thrown to Thread-A.  If Thread-B only modifies the
attributes of Element-5, then Thread-A will be unaware.  While this
won't throw an exception to Thread-A, it might radically change
the semantics of the document unbeknownest to Thread-A.  


If all threads *only* read, you're safe across the board.

If you want to read and write across a shared document, then I
suggest implementing a ReadWriteLock, associated with the Document
object, or similar, for maximum efficiency.


On Sat, 27 Oct 2001, Mark Bennett wrote:

> Sorry to be dense, but specifically, should simultaneous reading/traversal
> of a tree be OK?  As long as no modifications are made?
> -----Original Message-----
> From: Jason Hunter [mailto:jhunter at acm.org]
> Sent: Saturday, October 27, 2001 10:18 AM
> To: mbennett at ideaeng.com
> Cc: jdom-interest at jdom.org
> Subject: Re: [jdom-interest] Thread safe question: OK for simultaneous
> read-only access?
> > JDOM is generally not thread safe, as I understand it.
> True.  We follow the same model as ArrayList, which is not by default
> thread safe.
> > It's suggested that
> > it not be used in such an environment.
> Not true.  It's simply suggested you manage your own thread safety.  See
> the FAQ for an explanation.
> -jh-
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